First order of business would be avoiding electrocution Those capacitors hold power even when the system's unplugged. Unplugging the system and then hitting the Power button on the case may drain the capacitors, but still, be careful not to touch them. Take off any metal necklaces/bracelets that could get in there while you're working on it.
Next thing is getting the PSU out of the case and opening it, not too hard. Then you can have a look at how the fans are attached to the circuitry and determine what to do next.
If the fan plugs into the circuit board on a 2-pin header using a 2-pin plug, unplug it and look at the plug. It will probably have little rectangular holes as shown in the closeup shot at the bottom of
this page.
- First, take note of which wire is the positive wire on the 2-pin plug (AFAIK this wire is always red) and mark that side of the plug somehow.
- Next, take a saftey pin and press down on the little metal "catch" or "prong" visible through the rectangular holes, and gently pull the wire and contact out of the plug.
- Now remove the new fan's wires from its plug in the same way. If it has a third RPM wire (the Vantec fan will, and it's yellow), snip it off as close to the fan's motor as practical.
- Finally, put the new fan's power and ground wires into the 2-pin plug, and plug the fan back into the PSU's circuit board.
If the fan's wiring is soldered directly to the circuit board:
- Snip the wires that connect the existing fan to the circuit board, leaving at least three inches attached to the circuit board, and do the same with the new fans
- Slide pieces of appropriately-sized heatshrink tubing onto the wires attached to the fan
- Strip the insulation for about 1cm on the wires attached to both the fan and the circuit board
- Put some flux on them, then heat them a bit with a soldering iron, and tin them (apply a coat of solder)
- After tinning all four wires, solder them together (positive-to-positive, negative-to-negative of course), slide the heatshrink tubing over the joint and shrink it with heat from the soldering iron
Another option is to route the fans' stock wiring out the same hole that all the other wiring exits through, and then hook the fans up to a 3-pin-to-4-pin converter externally. This avoids having to do the mods I described, but you sure wouldn't want to accidentally leave them unplugged. Whatever the case, make sure the fans' wiring is ziptied out of the way so it can't get snagged in the fan blades.